In order to reduce capacitance and to electrically isolate devices, silicon-on-insulator (SOI) wafers are used in place of bulk Si wafers. One approach to forming an SOI wafer is to use separation by implantation of oxygen (SIMOX) where a buried oxide layer is formed in a wafer by implanting oxygen ions and then annealing at high temperatures.
An example of an advance SIMOX process is described in U.S. Pat. No. 5,930,643 which issued on Jul. 27, 1999 to D. K. Sadana and J. de Souza entitled “Defect Induced Buried Oxide” which describes implanting oxygen into a Si wafer at high temperature to form a stable defect region in the Si followed by implanting oxygen at a temperature below 300° C. to form an amorphous Si region adjacent the stable defect region.
U.S. Pat. No. 6,043,166 which issued Mar. 28, 2000 describes forming a high quality buried oxide (BOX) layer with extremely low doses of oxygen followed by two high temperature oxidation anneals to eliminate defects in the silicon above the buried oxide by forming silicon dioxide as part of the buried oxide in the region where the defects were present.
U.S. Pat. No. 6,090,689 which issued Jul. 18, 2000 describes forming Silicon-on-Insulator substrates incorporating the steps of ion implanting oxygen into a silicon substrate at an elevated temperature, ion implanting oxygen at a temperature below 100 degrees ° C. at a lower dose to form an amorphous silicon layer, and annealing steps to form a mixture of defective single crystal silicon and polycrystalline silicon or polycrystalline silicon alone from the amorphous silicon layer and then silicon oxide to form a continuous silicon oxide layer below the surface of the silicon substrate to provide an isolated superficial layer of silicon. The low temperature implant results in the formation of a buried amorphous layer at the location where the oxide is to be formed. The amorphous silicon layer contains both dissolved and precipitated oxygen which forms polycrystalline silicon to provide sites for nucleating oxide growth and paths for rapid diffusion of oxygen along the polycrystalline grain boundaries.